refactor(core): refactor WrappedValue (#20997)

- Improve `WrappedValue` by adding `unwrap` symetrical to `wrap`.
- remove dead code - `ValueUnwrapper`

The property `wrapped` is an implementation details and should never be accessed
directly - use `unwrap(wrappedValue)`. Will change to protected in Angular 7.

PR Close #20997

tools/public_api_guard/core/core.d.ts

@@ -1093,8 +1093,10 @@ export declare abstract class ViewRef extends ChangeDetectorRef {
 
 /** @stable */
 export declare class WrappedValue {
-    wrapped: any;
-    constructor(wrapped: any);
+    /** @deprecated */ wrapped: any;
+    constructor(value: any);
+    static isWrapped(value: any): value is WrappedValue;
+    static unwrap(value: any): any;
     static wrap(value: any): WrappedValue;
 }